linux-kernel - Re: [PATCH] kernel/cpu.c: Move the CPU

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Date:	Sun, 31 Aug 2008 12:23:09 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] kernel/cpu.c: Move the CPU_DYING notifiers

On Sun, Aug 31, 2008 at 12:17:21PM -0700, Paul E. McKenney wrote:
> On Sun, Aug 31, 2008 at 07:58:49PM +0200, Manfred Spraul wrote:
> > When a cpu is taken offline, the CPU_DYING notifiers are called on the
> > dying cpu. According to <linux/notifiers.h>, the cpu should be "not
> > running any task, not handling interrupts, soon dead".
> > 
> > For the current implementation, this is not true:
> > - __cpu_disable can fail. If it fails, then the cpu will remain alive
> >   and happy.
> > - At least on x86, __cpu_disable() briefly enables the local interrupts
> >   to handle any outstanding interrupts.
> > 
> > What about moving CPU_DYING down a few lines, behind the __cpu_disable()
> > line?
> > There are only two CPU_DYING handlers in the kernel right now: one in
> > kvm, one in the scheduler. Both should work with the patch applied
> > [and: I'm not sure if either one handles a failing __cpu_disable()]
> > 
> > The patch survives simple offlining a cpu. kvm untested due to lack
> > of a test setup.
> 
> Several architectures re-enable interrupts in __cpu_disable() or in
> functions called from __cpu_disable(), which happens after CPU_DYING,
> if I understand correctly.  :-(

Never mind -- you are moving CPU_DYING after __cpu_disable().  :-/

							Thanx, Paul

> > Signed-Off-By: Manfred Spraul <manfred@...orfullife.com>
> > ---
> >  kernel/cpu.c |    5 +++--
> >  1 files changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index e202a68..5b7c88f 100644
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -199,13 +199,14 @@ static int __ref take_cpu_down(void *_param)
> >  	struct take_cpu_down_param *param = _param;
> >  	int err;
> > 
> > -	raw_notifier_call_chain(&cpu_chain, CPU_DYING | param->mod,
> > -				param->hcpu);
> >  	/* Ensure this CPU doesn't handle any more interrupts. */
> >  	err = __cpu_disable();
> >  	if (err < 0)
> >  		return err;
> > 
> > +	raw_notifier_call_chain(&cpu_chain, CPU_DYING | param->mod,
> > +				param->hcpu);
> > +
> >  	/* Force idle task to run as soon as we yield: it should
> >  	   immediately notice cpu is offline and die quickly. */
> >  	sched_idle_next();
> > -- 
> > 1.5.5.1
> > 
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